In the manufacturing micro-devices (e.g., integrated circuits, thin-film head and ink jet heads) the processing steps include exposing a substrate, such as a semiconductor wafer coated with photosensitive material, using a lithographic exposure system. This exposure requires aligning the substrate residing on a substrate (wafer) stage, to a reticle having a pattern of a particular device layer, and residing on a reticle stage. In this regard, the lithographic system includes an alignment system, such as that disclosed in U.S. Pat. No. 5,621,813 (referred to hereinafter as “the '813 patent”), which patent is incorporated herein by reference. After alignment, the reticle is exposed to radiation to which the photosensitive coating is sensitive, to transfer the reticle pattern onto the wafer. This alignment and exposure can be performed on a variety of lithography systems such as step and repeat, projection, contact and proximity systems, for example. Typically, the first of such device layers is aligned to some marking on the wafer, for example, to a flat or notch, as is well known. Subsequent layers are then aligned relative to this first layer and/or to each other.
Most exposure systems utilize some mechanical means of pre-aligning the wafer, so that the wafer is coarsely aligned to the reticle. The pre-alignment may be, for example, a mechanical means of locating a flat or notch on the wafer. Alternatively, optical sensors may determine the location of the flat, notch, or peripheral edge of the wafer. These methods typically align the wafer to an accuracy of a few hundred microns. After mechanical pre-alignment, the wafer is moved to or near the exposure position by, for example, a wafer-handling arm. Often, after the above-described mechanical alignment, but prior to fine alignment, a pre-alignment using a photoelectric detector is performed. Special optical alignment targets (OATs), produced on the substrate by previous processing steps, are used for this purpose. The OATs are relatively large, so that they can be quickly found after the relatively coarse mechanical pre-alignment. This pre-alignment using the OATs typically aligns the wafer to within approximately ±50 microns or better. At this point, a fine alignment may be performed, by aligning alignment keys on the reticle to alignment targets on the wafer.
The alignment keys and targets are typically on the order of a few microns in size, and provide for alignment to a precision of, for example, 0.15 micron or less, depending upon the requirements of the user. The fine alignment can be performed via a photoelectric detector, such as photomultiple tube or CCD array, which can detect the superposition of special-purpose alignment marks on the reticle and wafer. Based upon the superposition signal level, the detection apparatus sends a signal to move the wafer and/or reticle stage such that the alignment targets on the wafer are in alignment with the alignment keys on the reticle.
The alignment system may be an off-axis system, wherein the wafer is aligned out of the exposure of field of the optical system, then moved to the exposure field with high accuracy to align the wafer to the reticle. Alternatively, the alignment can be performed “through the lens” (TTL) of the optical system. This is also called “on-axis” alignment, and the wafer remains in place during such alignment. Some off-axis alignment systems rely upon aligning the reticle to a mechanical reference built into the lithography system. The substrate is aligned to this mechanical reference as well, and thus to the reticle by commutation. However this scheme requires that the mechanical reference be frequently calibrated for the offset of the substrate to the reticle. Furthermore, very high mechanical stability is required. The TTL technique allow the examination of the actual superposition of reticle image and the substrate for alignment, thereby eliminating the need for a mechanical reference. A TTL system can be configured in a variety of ways, for example, the prior art method of using the projection lens (or a portion of the projection lens) to view the projection of the reticle image onto the substrate. Many alignment systems require scanning, that is, relative motion between the reticle and wafer, which introduces some error.
A problem in present-day lithography practice is that each lithography system has a particular hardware-dependent alignment offset, making it difficult and time-consuming to run different processes on different systems unless the offset is known for the particular tool. Making matters worse is the fact that each “job” has a job-dependent alignment offset. Here, a “job” denotes a different process step carried out on the machine that involves a different reticle to be exposed. Thus, if a particular lithography system used for a given step breaks down, an otherwise available lithography system cannot be used in its place without a great deal of inconvenience in characterizing the offset for the available machine for the particular job that needs to be run.
The prior art method of characterizing alignment offsets for a particular job being run on a particular lithography system (hereinafter, “machine”) involves running so-called “send ahead” wafers to measure the alignment offset for each machine and each job. With reference to FIG. 1A, the prior art method involves exposing a first pattern 10 with a center 10C, such as a fairly large box (e.g., 50 microns on a side), at a first location on the wafer. The wafer is then developed, and re-coated with photoresist and re-loaded into the machine. A second pattern 20 with a center 2C, similar or identical to that of pattern 10, is then printed on the wafer at a second location so that its center 20C is precisely displayed from center 10C by a predetermined distance (e.g., 200 microns). This displacement is accomplished by programming the machine to move either the wafer stage or reticle stage (or both) by the predetermined distance 30 (see FIG. 2). This “send ahead” wafer is then sent to an independent alignment measurement tool. The alignment measurement tool measures the precise location of centers 10C and 20C of patterns 10 and 20, respectively, relative to some reference point. From this information, the measurement tool deduces the actual displacement 30′ between the respective centers. For a perfect machine, the measured displacement 30 would be identical to that of the programmed displacement 30′. However, with reference to FIG. 1B, in practical actual measured displacement 30 and programmed displacement 30′ are different. This different Δ represents the “alignment offset” for the particular combination of machine and job.
The alignment offset can be due to a number of factors, such as differences in viewing at the alignment wavelength of light (which is visible or near-visible) to the actinic light (typically ultraviolet), mechanical calibration, and the sensitivity of the pattern recognition software to the particular alignment pattern (“mark”) printed on the wafer. More generally stated, the alignment offset has a machine-dependent hardware contribution and a job-dependent pattern contribution. Thus, with reference to FIG. 2, in order to perform a number of jobs (J1 to Jn) on a number of different machines M1 to Mn, a large number (n2) of offset measurements OM9 need to be performed (one measurement for each machine-job combination) to create a matrix of information relating each job to each machine.
It would be greatly advantageous to have a system for and method of calibrating a lithography system (“machine”) that allows a given process (“job”) to be run on any machine within a family of machines. A method and system for machine characterization that eliminates the need for time-consuming and costly send-ahead wafers would provide for much desired “job portability” in semiconductor manufacturing.